Crude nitric oxide is produced industrially by various known methods, such as the ammonia oxidation method, the reaction of sodium nitrite and ferrous chloride, or the reaction of nitric acid and sulfur dioxide. High-purity nitric oxide is produced by removing acid gas by means of adsorbent or alkaline aqueous solution and removing water and others by means of adsorbent from the crude nitric oxide. Nitric oxide having a purity of at least 99.95% in a state of being filled in a high-pressure gas cylinder is commercially available in recent years for the medical therapy use, semiconductor material, and so forth.
Such nitric oxide is a thermodynamically unstable material, so that reduction of purity is caused by disproportionation reaction occurring partially in the high-pressure gas cylinder. To be more precise, the disproportionation reaction represented by the following reaction equation (1) occurs in the high-pressure gas cylinder, so that the nitric oxide is converted into nitrous oxide N2O) and nitrogen dioxide (NO2) that is equimolar to the nitrous oxide.3NO→NO2+N2O  (1)
Non-patent Document 1 discloses that the production rate of nitrous oxide and nitrogen dioxide due to the disproportionation reaction at pressures of 200 atm and above is represented by the following equation (2), and the disproportionation rate increases when the temperature or pressure is increased.−d[NO]/dt=k′[NO]3=d[N2]/dt=d[NO2]dt  (2)
Here, k′ is the reaction rate constant, d/dt is the time derivative, and [NO], [N2O], and [N2O] are the molar concentrations of nitric oxide, nitrogen dioxide, and nitrous oxide respectively.
Non-patent Document 2 discloses the relationship between the disproportionation reaction of nitric oxide and the temperature in a high temperature region from 700° C. to 1800° C.
Non-patent Document 3 discloses the amounts of nitrous oxide and nitrogen dioxide that are produced, for example, when nitric oxide is left to stand for 1 month at 298 K and 10 atm.